Phosphorous concentration in iron-rich rocks of the Chilpi Group, Bastar Craton, India: implications on late Palaeoproterozoic seawater palaeo-productivity

The concentration of the bio-limiting nutrient element, phosphorus (P), in seawater is important for primary marine productivity and the evolution of life on geological time scales. The molar percentage of P/Fe in banded iron formations (BIF) and iron oxide-rich chemical sediments is a good proxy for the first-order approximation of seawater P concentration. Bio-available concentration of phosphorus in Precambrian, especially during the late Palaeoproterozoic Era (2.0-1.8 Ga), is poorly constrained. We evaluated the P/Fe ratios of iron-rich rocks from the late Palaeoproterozoic Chilpi Group, Bastar Craton, Central India. The bulk rock molar percentage of P/Fe ratios of the Chilpi rocks vary between 0.11 and 1.17 (average 0.51 +/- 0.3), and the average of EPMA spot analysis P/Fe molar ratio is 0.32 +/- 0.4; both have values similar to Archaean BIFs of the world. The observed low molar ratio is not an artefact of contamination from terrestrial sources, diagenetic alterations or high-temperature hydrothermal inputs; it indicates the deposition from phosphorus-lean seawater. The modelled P/Fe molar ratio in the Chilpi Group suggests that the concentration of phosphorus in the shallow marine environment was less than 0.12 mu M. The low level of phosphorus concentration in seawater during the late Palaeoproterozoic Era is interpreted to be a consequence of the low primary production during a period of low atmospheric oxygen content, which might have impeded the evolution of eukaryotes.

Automated summary

The concentration of phosphorus in seawater is important for marine productivity and life evolution. By studying the P/Fe ratio in iron-ore rocks and iron oxide-rich sediments, we can estimate the phosphorus concentration in seawater. In this study, we evaluated the P/Fe ratios of iron-rich rocks from the late Palaeoproterozoic Chilpi Group in Central India, and found that the phosphorus concentration during this period was low. This low phosphorus concentration may have hindered the evolution of eukaryotes due to reduced primary productivity caused by low atmospheric oxygen content.